1. Field of the Invention
The present invention relates to an AC generator having a full-wave-rectifying-circuit and particularly relates to an alternator for charging a battery installed in a vehicle or the like and a generator or a motor-generator for an electric vehicle.
2. Description of Related Art
An alternator exemplified in FIG. 32 is equipped with a rotor 110 which has a field coil 100 and a stator (not shown) which has a three-phase-armature coil 120. When the rotor (magnetic field) 110 rotates with the field coil 100 being excited by an exciting circuit 130, respective phase-voltages are induced in the respective phase-windings 120a, 120b and 120c. These voltages are converted (rectified) by a full-wave-rectifying-circuit having six-diodes 140 into a DC-current, which is supplied to a battery 150 and an electric load 160.
The alternator is required to generate more electric power (especially, power during a low speed range of an engine) since the electric load of the vehicle is increasing. Measures to increase the generating power are as follows: (1) increasing the alternator-size, (2) increasing the field current and (3) selecting a number of turns of the armature coil according to the rotational speed of the alternator.
Since the space of the engine compartment has been narrowed recently, a decrease in alternator size as well as an increase in output power has been required.
Although one of above measures --(1) increasing the alternator size-increases the output power over the entire speed range from low speed to high speed as shown in FIG. 33, it does not comply with the downsizing requirement. Although the second measure--(2) increasing the field current-increases the output power in the high speed range as shown in FIG. 34, it does not increase the output power in the low speed range. Although the third measure--(3) selecting the number of turns of the armature coil according to the rotational speed of the alternator-increases the output power in a high speed range when the number of turns of the armature coil is decreased as shown in FIG. 35, it decreases the output power in the low speed range. On the other hand, in order to increase the output power in the low speed range, if the number of armature-coil-turns is increased, the body-size will increase and the output in the high speed range will decrease as shown in FIG. 35.
Thus, it is rather difficult for the conventional alternator to increase the output power, particularly in the low rotational speed range, without increasing the body-size. Therefore, an entirely new idea is necessary for that purpose.
FIG. 36 is a circuit diagram of a single phase model illustrating the principle of the electric power generation.
The single phase model is composed of an armature resistance r.sub.a, an armature inductance L, a load resistance R, and an AC power source (induced-phase-voltage E.sub.0). Accordingly, a voltage V across the resistance R lags behind the induced-phase-voltage E.sub.0 by a phase-angle difference .delta. as shown in FIG. 37 and FIG. 38.
The phase-angle difference .delta. is necessarily fixed by the circuit constants R, R.sub.a and .omega.L as is given by the following equation. EQU .delta.=tan.sup.-1 {.omega.L/(R+r.sub.a)}, [E1]
wherein
.omega.: electric-angular-velocity [.omega.=(p/2).times.(n/60).times.2.pi.] PA1 p: the number of poles PA1 n: rotational speed (rpm) PA1 Z.sub.s : synchronous impedance [Z.sub.s =.sqroot. {(.omega.L).sup.2 +r.sub.a.sup.2 }]
On the other hand, a phase-current flowing through the resistance R is given by the following equation. EQU I=(E.sub.0 -V cos .delta.)/Z.sub.s [E 2]
Since the frequency is low during the low speed range, the electric-angular-velocity .omega., and, consequently, .omega.L become small, and thus the phase angle difference .delta. becomes small according to the equation [E1]. That is, the phase lag of the voltage V relative to the induced-phase-voltage E.sub.0 becomes smaller as the rotational speed becomes lower.
As the rotational speed becomes lower, in other words, the phase angle difference .delta. becomes smaller, cos .delta. in the equation [E2] becomes greater, and accordingly, the phase current I, which flows through the load resistance R, becomes smaller and the output power P becomes smaller as shown in FIG. 39.
From the above study, if the phase angle difference .delta. between E.sub.0 and V is increased during the low speed range, the current I will increase so that the total output current increases and, consequently, the output power of the alternator will increase.
Accordingly, if an AC-voltage which lags behind the induced-line-voltage (or induced-phase-voltage E.sub.0) by a certain phase angle is applied across an adjacent couple of phase-windings to compose a phase-voltage which lags behind the induced-line-voltage (or induced-phase-voltage E.sub.0), the phase angle difference .delta. between E.sub.0 and V will increase.